This invention relates to a defect inspection method of inspecting internal defects of a glass substrate for a mask blank, a glass substrate for a mask blank to be inspected by the defect inspection method, a mask blank produced from the glass substrate, an exposure mask produced from the mask blank, a method of producing a glass substrate for a mask blank after inspecting internal defects of the glass substrate, a method of producing a mask blank by the use of the glass substrate, and a method of producing an exposure mask by the use of the mask blank.
In recent years, following a miniaturization of a semiconductor device, an exposure light having a shorter wavelength is used in the photolithography. For example, an ArF excimer laser (having an exposure wavelength of 193 nm) and a F2 excimer laser (having an exposure wavelength of 157 nm) are used as the exposure light. In an exposure mask used in the photolithography and a mask blank used to produce the exposure mask, development is rapidly carried out of a light shielding film (opaque film) having a light shielding function for the exposure wavelength of the exposure light and a phase shift film having a phase shifting function and various film materials are proposed.
It is required that the above-mentioned glass substrate for a mask blank and a synthetic silica glass substrate used for producing the glass substrate for a mask blank do not have a defect, such as a foreign matter or an air bubble, present inside. Japanese Unexamined Patent Application Publication (JP-A) No. H8-261953 discloses a defect inspection apparatus for detecting internal defects (such as foreign matters or air bubbles) present in a glass substrate by introducing a He—Ne laser beam into the glass substrate and detecting a scattered light scattered by the internal defects.
However, even if the exposure mask is produced by the use of the synthetic silica glass substrate and the glass substrate for a mask blank which are judged by the above-mentioned defect inspection apparatus to have no internal defect, a transfer pattern defect derived from the glass substrate (which will later be described) may be caused during pattern transfer in which a mask pattern of the exposure mask is transferred to a semiconductor substrate by the use of an ArF excimer laser as the exposure light.
This is presumably because some internal defects (such as local cords (local striae), inclusions, heterogeneous matters) present in the glass substrate locally change optical characteristics (for example, decrease transmittance) when a high-energy light such as an ArF excimer laser beam or a F2 excimer laser beam is used as the exposure light although no scattering occurs when a visible light laser beam such as a He—Ne laser beam is used as the exposure light.